An individual's health or fitness can be assessed from the perspective of energy expenditure over time. Two techniques for estimating energy expenditure, or calorie burn, are described in the art. The first is based on measuring the physiologic and metabolic responses to exercise, while the second is based on measuring the physical work the body performs moving a defined load, or resistance, through space.
Physiological estimates of energy expenditure are most commonly based on heart rate. During moderate to vigorous exercise, heart rate is correlated with energy expenditure. At a macroscopic level, an individual's heart rate indicates how quickly the individual's body is delivering oxygen to vital organs and tissues, which consume the oxygen through oxidative cellular metabolism. The heart pumps blood through the lungs, where red blood cells absorb oxygen from the lungs. This oxygen-rich blood returns to the heart, from which it is pumped through blood vessels that distribute the blood throughout the body to its organs and tissues. Tissues absorb oxygen carried by the blood and use the oxygen in chemical reactions of oxidative metabolism, also known as aerobic metabolism, to provide energy for biological functions.
The rate at which an individual body consumes oxygen at a given point in time is referred to as the volumetric flow of oxygen into the tissues of the body, also known as “oxygen exchange rate,” “oxygen uptake rate,” or simply {dot over (V)}O2 (e.g., liters of oxygen per minute). Controlling for differences in body size, {dot over (V)}O2 is often reported for a given individual in terms of oxygen volume at standard temperature and pressure per unit of time per unit of body mass (e.g., ml/kg/min).
Specifically, {dot over (V)}O2 measures the overall rate at which the body is engaged in oxidative metabolism. {dot over (V)}O2 during various physical activities—and, consequently, energy expenditure during those physical activities—varies from individual to individual. In a laboratory setting, it may be possible to use indirect calorimetry (e.g., with a face mask that measures the rate of oxygen consumption and rate of carbon dioxide production), to measure an individual's aerobic capacity, also known as maximum {dot over (V)}O2, or simply “{dot over (V)}O2max.” {dot over (V)}O2max is the highest rate of oxygen exchange that an individual can sustain.
In addition to {dot over (V)}O2max, several other parameters are used to estimate an individual's energy expenditure at a given heart rate. Maximum heart rate (HRmax) and resting heart rate (RHR) are two such examples. An individual's heart rate generally will not exceed a maximum value, and, during exercise, the individual will reach this heart rate at their maximum energy output. Similarly, the resting heart rate is the value obtained when the user is completely at rest, and corresponds to an energy expenditure equal to that needed to sustain only basic physiological processes. Heart rates between RHR and HRmax are achieved at energy expenditures between these two extremes.
Most forms of aerobic exercise involved the repetitive application of muscular forces against a combination of intrinsic or extrinsic resistance forces (collectively, the load). For example, running involves repetitive gait cycles in which physical work is performed to accelerate the legs/arms around their respective joints (intrinsic load) while also propelling the body's center of mass against the resistive forces of air drag and gravity (extrinsic load). An alternative method of measuring energy expenditure therefore involves measuring the rate at which physical work is performed against a known or implied load.
Since the two general methods of energy expenditure should be equal over sufficiently long time scales, combinations of the two methods can be used to calibrate an unknown parameter. For example, during outdoor running where work rate can be accurately measured and internal resistive forces assumed constant, heart rate measurements can be used to calibrate a user's unknown {dot over (V)}O2max. Conversely, for user's with a known VO2max exercising against a variable resistance, work rate measurements may be used to infer the unknown load.
A problem with existing methods is that load and {dot over (V)}O2max are never perfectly known; therefore, existing calibration methods introduce bias by ignoring the effect of uncertainty in the value of one assumed parameter on the estimation of the other parameter. In addition, the requirement that either load or {dot over (V)}O2max be known in advance limits the application of these calibration methods to specific users and specific exercise scenarios.